1. Field of the Invention
The present invention relates to an apparatus for detecting a fully closed state of a throttle valve of an engine, and more particularly, to a throttle-opening detecting apparatus which can judge if the throttle valve is in its fully closed state based on the output of a throttle-opening sensor.
2. Description of Related Art
As a general apparatus for detecting idling of engine, there is known a throttle opening detecting apparatus disclosed in Japanese Utility Model Laid-Open No. 59-52045. The apparatus is equipped with an idle switch which is turned on when the throttle valve is fully closed, and when the idle switch is operated, it is judged that the engine is in the idling state. However, a structure such as this has the disadvantage that the cost of production increases, because it requires the idle switch and the wiring for the switch.
Also, disclosed in Japanese Patent Publication No. 63-15467 and Japanese Patent Laid-Open No. 4-241762 are idling-state detecting apparatuses which learn and store, with the use of a microcomputer, the fully closed state of a throttle valve detected by the throttle-opening sensor at the time of predetermined conditions where the throttle valve is fully closed, and judge that the throttle valve is in the fully closed state, when the difference between the stored learned value and the detected value of the throttle-opening sensor becomes less than a predetermined value.
A conventional throttle-opening detecting apparatus will hereinafter be described in reference to FIGS. 6 and 7.
In FIG. 6, an engine 1 has a plurality of cylinders (only one cylinder shown) each provided with a combustion chamber 2. An intake pipe 3 is connected with the combustion chamber 2 for supplying thereto intake air or a fuel/air mixture. The intake pipe 3 is provided with an air flow sensor 5 for sensing a flow rate of intake air sucked into each combustion chamber 2 via the intake pipe 3. A throttle valve 6 is disposed in the intake pipe 3 and moved to open and close for controlling the flow rate of intake air through the intake pipe 3 in response to an accelerator pedal (not shown). A surge tank 7 is connected to the intake pipe 3 for stabilizing the quantity of an intake air supplied to each cylinder through the intake pipe 3.
An intake manifold 8 is connected at its one end with the surge tank 7 and at its other ends with the respective combustion chambers 2 of the cylinders. A fuel injection valve 9 for injecting fuel into intake air is mounted in the intake manifold 8 so that an injection nozzle of the injection valve 9 is directed toward a downstream side of the intake manifold 8. In addition, in order to control an idle intake quantity at the time when the degree of opening of the throttle valve 6 is of a minimum value, i.e., at the time of idling, an idle intake pipe 10 of a small diameter is connected with the intake pipe 3 so as to bypass the throttle valve 6. The degree of opening of the idle intake pipe 10 is controlled by an idle intake control valve 11, which is provided in the idle intake pipe 10, so that the engine speed at the time of idling is maintained at a predetermined value. Furthermore, there are provided a throttle-opening sensor 13 for sensing the degree of opening of the throttle valve 6 and an engine-speed sensor 14 for sensing the rotational speed of the engine.
The air flow sensor 5, the fuel injection valve 9, the idle intake control valve 11, the throttle-opening sensor 13, and the engine-speed sensor 14 are all connected to a engine control unit 12 which in turn controls the fuel injection valve 9 and the idle intake control valve 11 based on output signals from these sensors.
Now, the throttle-opening sensor and a fully-closed state detector will be described in reference to FIG. 7. The throttle-opening sensor 13, shown in FIG. 7, outputs voltage proportional to the degree of opening of the throttle valve 6 which is opened and closed in response to the operation of the accelerator pedal. A general internal structure of the throttle-opening sensor 13 is the same as a potential meter, as shown in FIG. 7, and a slidable contact 13b moves on and along a resistor 13a in synchronization with the throttle valve 6, whereby a divided voltage is output from an output terminal 13c. The opposite ends of the resistor 13c are connected to a power supply terminal and a ground terminal, respectively.
Here, it is to be noted that the range between the opposite ends of the resistor 13a is generally set greater than the range where the movable contact 13b is moved by rotation of the throttle valve 6.
The fully closed state of the throttle valve 6 is detected by the fully-closed state detector which is constituted by a microcomputer, etc. The fully-closed state detector 15 takes in the output voltage of the throttle-opening sensor 13 through the output terminal 13c and also takes in various states of the engine, for example, the engine speed detected by the engine-speed sensor 14, the air flow sensor output, etc. Based on these outputs, the fully-closed state detector judges whether a predetermined condition is established, and learns and stores the degree of opening of the fully closed state of the throttle valve 6 based on the output value of the throttle-opening sensor when the predetermined condition is established.
A fully-closed state judging value is obtained by adding a predetermined value to the learned value of the degree of opening of the fully closed state of the throttle valve stored. If the output value of the throttle-opening sensor is less than the fully-closed state judging value, the fully-closed state detector 15 will judge that the throttle valve is in the fully closed state.
However, a conventional detector such as described above has following problems.
In general, in the case where the throttle valve is opened and closed, the output value of the throttle-opening sensor does not become the same value even when the throttle valve is fully closed. This is because the output value is influenced by the structures, the materials, and the temperatures of the throttle valve and the throttle-opening sensor. As a structural influence of the throttle-opening sensor, it can be given that the positions of the throttle valve and the throttle-opening sensor differ each time the throttle valve is fully closed. The reason that the values of the fully closed states thus differ in that every full-closing operation is that the throttle valve and the throttle-opening sensor are a rotating body and they require play on an axis of rotation.
In addition, for a reason such as this, the output value of the throttle-opening sensor at the time of the fully closed state varies depending upon the speed when the throttle valve is closed or a change with the passage of time.
Therefore, in the conventional fully-closed state detector, in order to correctly judge the fully closed state of the throttle valve, a predetermined fixed value for judging the fully closed state must be set in view of all variations in the output value of the fully closed state of the throttle-opening sensor resulting from variations in the structure, material, temperature, and changes in settings of the throttle valve and the throttle-opening sensor over time, and hence the judgment value is set to a value greater than an actual value of the fully closed state. For this mason, the number of cases, where the throttle valve is judged as being in the fully closed state although it is open, is increased in the judgment of the fully closed state which uses the aforementioned fully-closed state judging value.
For this reason, in the idle speed control for maintaining engine speed constant, which is performed, for example, at the time of idling of the engine, the engine speed at the time of the fully closed state of the throttle valve (at the time of idling) must be fed back as one of the conditions for feeding back the engine speed and controlling a quantity of intake air of the engine. When, under such a condition, the throttle valve is judged as being in the fully closed state although it is open, as described above, control is performed such that the effective cross sectional area of the bypass passage is reduced by the idle intake control value 11. When the throttle valve is fully closed in such a situation, the engine speed is reduced because the bypass passage has been narrowed, and in the worst case there is the possibility that the engine stalls. In addition, such a mistaken detection of the fully closed state of the throttle valve constitutes an obstacle to the other controls, for example, all the controls in which switching is effected based on whether the throttle valve is fully closed, such as fuel control, ignition timing control, and automatic speed change control, and consequently, there is the problem that the controllability of the engine gets worse.